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Started on GPT-6-Sol, then moved to GPT-6-Sol

Astronomy studies celestial objects and phenomena, including stars, planets and galaxies. Astrometry measures their positions and motion; astrophysics uses physics to explain their properties and behaviour.

Much of astronomy begins with incoming light. An image tells us which direction the light arrived from, rather than directly providing the source's three-dimensional position. Two stars close together on the sky may be very far apart in space. The celestial sphere organizes these viewing directions; locating an object on it is a different task from measuring how far away that object is. Repeated observations turn this limitation into an opportunity: the changing direction of light contains evidence about both Euclidean distance and motion.

The particularly satisfying combination is stellar parallax with proper motion. The first supplies a geometric Euclidean distance, while the second measures how rapidly the viewing direction changes after accounting for the observer's annual motion. Together they give a transverse speed, connecting tiny angles on an image to kilometres travelled each second. This bridge from a flat sky map to moving stars is one reason astrometry matters to the study of the Milky Way.

The map from a three-dimensional astronomical distribution to its observed sky position or integrated surface signal. For a scalar volume emissivity $j(\mathbf{x})$ in simple transparent geometry, surface brightness is proportional to $\int jdl$ along the ray. Reconstructing the spatial distribution requires additional geometry or symmetry assumptions; a projected surface distribution is not a unique three-dimensional density.

Scientific collaboration responsible for analysis of the Planck mission data.

A solar eclipse occurs when the Moon obscures part or all of the Sun as seen from a specified location. Totality allows observations of nearby background stars that ordinary daylight hides.

Astronomer and first author of a 1984 cold-dark-matter galaxy-formation model.

Astronomer and author of research on X-ray plasma spectroscopy.

Radio astronomer and first author of a Very Long Baseline Array solar-deflection measurement published in 2009.

An astrophysicist studying stellar black holes and X-ray binaries.

An astronomer studying star-forming galaxies, active galactic nuclei and galactic outflows.

An astrophysicist studying galaxy formation and cosmological gas accretion.